1. Field of the Invention
The present invention relates to an automatic transmission, and more particularly, to an engagement control for a friction device of an automatic transmission.
2. Background Art
An automatic transmission for a motor vehicle has a planetary gear set constituted of sun gears, ring gears, pinions and the like. The speed change of the transmission is accomplished by engaging or releasing friction devices such as multiple-disc clutches or multiple-disc brakes. Generally, the friction devices comprise a plurality of drive plates connected to an input side and a plurality of driven plates connected to an output side disposed in an interleaving relation to the drive plates. When a pressure of a pressure plate is applied to those drive and driven plates, power is transmitted from the drive plates to the driven plates through a frictional contact therebetween.
Besides these multiple-disk type friction elements, a one-way clutch is used for engaging or releasing gears in some type of automatic transmissions. For example, in a sport-shift type automatic transmission whose gear selection can be performed manually, the one-way clutch is locked to engage a first speed gear together with an concurrent engagement of a multiple-disk brake.
FIG. 8 is a cross sectional view showing a construction of a one-way clutch with a multiple-disk friction element according to a prior art. The one-way clutch comprises an inner race 71, a hub (outer race) 72 and a plurality of sprags 73 disposed between the inner race 71 and the hub 72. When the sprags 73 are arranged as shown in FIG. 8, in case where the hub 72 rotates in a locking direction A (clockwise direction in the drawing), for example in case of acceleration at 1st speed, the sprags 73 rise to engage the inner race 71 with the hub 72, thereby the rotation of the hub 72 is locked. On the other hand, in case where the hub 72 rotates in a couterclockwise direction (in a contrary direction to A), for example in case of deceleration at 1st speed, the sprags 73 fall sideways to allow the rotation of the hub 72. Further, drive plates 75 and driven plates 76 are arranged interchangeably in an axial direction of the hub 72. The drive plates 75 are spline-fitted to the hub 72 and the driven plates 76 are spline-fitted to a transmission housing 74. Thus, a brake mechanism is formed by the hub 72, the transmission housing 74 and the friction plates 75, 76. The inner race 71 is fixed to the transmission housing 74.
Considering a case where the hub 72 rotates in the locking direction A in a condition of the brake applied, first when the hub 72 rotates in the locking direction A, the sprags 73 start rising. Note that the hub 72 is not locked unless the sprags 73 rise. That is, the locking force of the one-way clutch is generated by rotating the hub 72 by a rotation amount necessary for raising up the sprags 73. Accordingly, the rotation of the hub 72 is restricted by either locking force of the one-way clutch or the one of the brake. As a result of this, sometimes the braking or locking force of the brake operates before the locking force of the one-way clutch is generated.
That is, as shown in FIG. 8, when the hub 72 rotates in the direction A, the clearance "a" (so-called backlash) in the fitting portions between the hub 72 and the drive plate 75 is closed so as to rotate the drive plate 75 in the direction A. At this moment, since the drive plate 75 is engaged with the driven plate 76, the driven plate 76 is also rotated in the direction A. Accordingly, the clearance or backlash "b" in the fitting portions between the driven plate 76 and the transmission housing 74 is also closed and the protrusions of the driven plate 76 abut against the tooth face of the spline. Thus, a load according to the torque of the hub 72 and the degree of the engagement of the plates 75 and 76 is applied to the protrusions of the friction plates 75, 76. When the vehicle makes a sudden start, since this load becomes very large, the driven plate 76 is fatigued to lose endurance. On the other hand, in order to enhance the endurance or strength of the friction plates, the use of a material of high grade provides the automatic transmission with an increase in cost.